In recent years, a wide-gap semiconductor substrate has attracted much attention as a semiconductor material. Because a wide-gap semiconductor substrate has the feature of having a small crystal lattice constant and a large band gap as compared with a silicon substrate and a gallium arsenide (GaAs) substrate which are conventionally widely used, and therefore has good physical properties, it is expected to be applied to fields which cannot be covered by a silicon substrate or a GaAs substrate. The wide-gap semiconductor substrate generally comprises a compound including an element in the second period of the periodic table of the elements, such as carbon (C), nitrogen (N) and oxygen (O), and, as examples thereof, mention may be made of silicon carbide (SiC), zinc oxide (ZnO), and the so-called group III-V compounds such as gallium nitride (GaN), aluminum nitride (AlN), boron nitride (BN) and boron phosphide (BP).
However, as described above, silicon carbide or the like which is used as a wide-gap semiconductor substrate has a small crystal lattice constant as compared with silicon or the like, that is, has a strong interatomic bond, and therefore a silicon carbide substrate has the disadvantage that it has a difficulty in being etched as compared with a silicon substrate because it is difficult to break the interatomic bond. Therefore, as a method of plasma etching such a wide-gap semiconductor substrate, the applicant have suggested the plasma etching method disclosed in the Japanese Unexamined Patent Application Publication No. 2011-096700.
In this plasma etching method, the object to be etched is a substrate having a structure in which a silicon dioxide (SiO2) film as mask is formed on a silicon carbide substrate which is a type of wide-gap semiconductor substrate, an inert gas such as He gas is supplied into a processing chamber and plasma is generated from the inert gas, thereby generating ions coming from the inert gas, and a bias potential is applied to a platen on which the silicon carbide substrate is placed to make the generated ions incident on the silicon carbide substrate, thereby heating the silicon carbide substrate to a predetermined etching temperature within a range of 200° C. to 400° C. Thereafter, an etching gas such as SF6 is supplied into the processing chamber and plasma is generated from the etching gas, thereby generating ions and reactive species, and a bias potential is applied to the platen and thereby the silicon carbide substrate is etched by sputtering by the generated ions and by a chemical reaction with radicals in a state where the temperature of the silicon carbide substrate is maintained at the etching temperature.
According to this plasma etching method, it is possible to supply part of the energy necessary for breaking the bonds between the silicon (Si) and the carbon (C) which form the silicon carbide substrate by heating the silicon carbide substrate placed on the platen to a predetermined etching temperature, and this makes it easy to break the interatomic bonds of the silicon carbide substrate, and therefore the silicon carbide substrate can be easily etched, and further can be etched with high accuracy.